Nasal continuous positive airway pressure (CPAP) is the nonsurgical treatment of choice for obstructive sleep apnea syndrome (OSAS). Its efficacy for eliminating apneas and hypopneas is well established, but there continue to be significant questions about the consequences for daytime functioning of regular vs. irregular CPAP use. The project addresses these clinical issues, building on results from our two recent studies. In the first, a microprocessor-monitor inside the CPAP machines of 35 OSAS patients revealed that only 46% of them actually used CPAP regularly (>= 4 hrs/night on >= 70% of nights). Compared to irregular CPAP users, regular users were more likely to report post- treatment improvements is daytime energy, suggesting that CPAP use may be associated with the magnitude of improvement in hypersomnolence experienced by patients. This protocol is designed to test the hypothesis that regular CPAP use results in significant improvements in objective measures of daytime sleepiness relative to irregular use. However, we do not expect regular CPAP use to yield daytime functioning comparable to that of age and sex-matched controls, because in a second study we found that CPAP significantly improved objective and subjective measures of daytime sleepiness, but the level of functioning appeared to be suboptimal, and as soon as CPAP was withdrawn, hypersomnolence returned to pre-treatment levels. Thus we seek to answer basic clinical questions regarding the effectiveness of CPAP for relieving hypersomnolence, relative to the regularity of its use and the duration of sleep when it is used. Such answers should improve our ability to effectively treat OSAS patients. We also seek to continue to refine the techniques we have developed for assessment of CPAP use, ambulatory motility, daytime vigilance, mood, and functional status, which will be needed for future studies on the epidemiology and natural history of OSAS and its treatment.